1. Field of the Invention
This invention relates to infrared fibers and more particularly, it is concerned with infrared transmitting fibers consisting of crystalline metal halides and an improved process for the production of the same.
2. Description of the Prior Art
Various efforts have hitherto been made to prepare infrared fibers by processing optical crystals as described in "Nikkei Electronics" Dec. 8, 1980, page 140.
Above all, to guide light in a fiber, there have been proposed (1) a fiber structure of loose clad type as disclosed in Japanese Patent Application OPI (Kokai) No. 145645/1978, in which an extruded fiber is loosely inserted in a tube, the crystalline fiber being used as a core and air being used as a cladding and (2) a fiber structure of step index type as disclosed in Japanese Patent Application OPI (Kokai) No. 143242/1979, in which the circumference of a core crystal under molten state is coated with a glass with a large viscosity and subjected to spinning to form a fiber clad with the glass. In addition, U.S. Pat. No. 4253731 describes a fiber structure (3) of step index type prepared by extruding simultaneously a core crystal and a cladding crystal.
The above described infrared fiber (1) is prepared as follows. Firstly, a core crystalline fiber is prepared by means of an apparatus as shown in FIG. 1. That is to say, core crystal 1 for extrusion billet is charged in extruding container 3 provided with die 4, extruding ram 6 and die holder 5. Core crystal 1 is heated at a predetermined temperature by heater 7 and then extruding ram 6 is pressed by a hydraulic press to form core crystal fiber 2. The thus formed core crystal fiber 2 is loosely inserted in polymer pipe 8 as shown in FIG. 2, (a) of FIG. 2 being a cross sectional view and (b) being a longitudinal sectional view. Polymers such as teflon are not suitable for transmission of a large input laser beam because of a large absorption loss of infrared rays resulting in a large loss at the contact area of the fiber and polymer pipe.
The structure (2) is mechanically weak due to the difference in thermal expansion between the crystalline core material and glassy cladding material, resulting in strain in the fiber. Moreover, many glassy materials are not suitable for the transmission of infrared rays of long wavelengths and produce a large transmission loss as a cladding layer of an infrared fiber. Thus, selection of a suitable cladding material is considerably restricted.
In the case of (3), the method of coating a core crystal with a cladding crystal comprising extruding simultaneously the core crystal, and cladding crystal has the disadvantage that it is difficult to obtain a fiber with a smaller diameter by processing the core to a great extent in a circular form because the interface between the core and cladding tends to be disordered.